Fiber optic systems have become widely used for high capacity telecommunications networks. In fiber optic systems, data is typically transmitted as a stream of light pulses, within an optical spectrum covering some range of optical frequencies around a central frequency. Such a stream of pulses is known as a “channel”. The capacity of fiber optic communications systems has been increased both by increasing the data rate for each channel, and by multiplexing channels at different wavelengths onto a single optical fiber (known in the art as Wavelength Division Multiplexing, or WDM). Future fiber optic networks are also envisioned to be “agile”, with the capability of adding and dropping optical channels at intermediate nodes in a network, and dynamically reconfiguring the optical paths through the network taken by each channel. These advanced networks require careful management of the distortions to optical pulses caused by propagation through optical fibers.
An optical pulse propagates through an optical system at a velocity known as the group velocity. The time delay for a pulse to propagate through an optical system is known as the group delay. For optical fibers, the group velocity varies with wavelength, such that the longer wavelength components of an optical pulse propagate slightly faster or slower (depending on the sign of the chromatic dispersion) than the shorter wavelength components. This typically leads to a broadening in time of an optical pulse propagating through an optical fiber. This broadening is known as chromatic dispersion. As the pulses broaden, they eventually overlap in time, and can no longer be distinguished at an optical receiver. Thus, chromatic dispersion represents one of the fundamental limitations to the maximum data rates and transmission distances which can be achieved in a fiber optic communications system.
In the art, chromatic dispersion D is conventionally defined as the derivative of group delay τg with respect to wavelength λ: D=dτg/dλ. Group delay is in turn defined as the negative of the derivative of optical phase φ with respect to frequency ω: τg=−dφ/dω. Chromatic dispersion is conventionally expressed in units of ps/nm, and group delay in units of ps.
In order to increase the data rates and transmission distances in a fiber optic network, one or more chromatic dispersion compensator devices are typically included in the network. The chromatic dispersion compensator is designed to create a chromatic dispersion opposite in sign and at least approximately equal in magnitude to the chromatic dispersion of a segment of the network. The function of the chromatic dispersion compensator is to undo the pulse distortion caused by propagation through the fiber optic network
Chromatic dispersion compensator devices can be broadly classified as either single or multi-channel, and either fixed or tunable. Single channel devices compensate chromatic dispersion for a single optical channel, while multi-channel devices operate simultaneously on a plurality of channels. For fixed chromatic dispersion compensators, the amount of chromatic dispersion is fixed at the time of manufacture or installation, while for tunable dispersion compensators, the amount of dispersion may be dynamically adjusted during operation of the network.
For WDM fiber optic systems, it is desirable to have multi-channel dispersion compensators, in order to avoid the cost and complexity of demultiplexing the WDM channels and routing each channel through a separate single channel dispersion compensator.
In general, the required amount of chromatic dispersion compensation may vary from one WDM channel to another, due for example to the variation of chromatic dispersion with wavelength for optical fibers, or due to a different routing through the fiber optic network for different optical channels. The required amount of chromatic dispersion compensation may also vary with time, due for example to changes in the chromatic dispersion of the fiber links with ambient temperature, or due to dynamic re-routing of optical channels through the network. Thus, it is desirable to have a chromatic dispersion compensator that can impart separately and dynamically adjustable amounts of compensation to each channel.
U.S. Pat. No. 5,166,818 issued to Chase et al. discloses a dispersion compensating device based on the principle of spatially separating the frequency components of a single optical channel, applying a phase correction to the frequency components, then recombining the frequency components. The application of this device is limited, since it operates only on a single optical channel, and because the optical passband narrows as the amount of chromatic dispersion increases.
U.S. Pat. No. 6,392,807 issued to Barbarossa et al. discloses a tunable chromatic dispersion compensator based on the Virtually Imaged Phased Array (VIPA). This device is capable of simultaneously compensating chromatic dispersion for a plurality of channels, but it is limited in application by the fact that it cannot adjust the chromatic dispersion of each channel separately. Instead, the tuning mechanism adjusts the chromatic dispersion of all channels together.
It is an object of the present invention to provide a chromatic dispersion compensator that would perform simultaneous chromatic dispersion compensation for multiple channels, with separately adjustable compensation for each channel. A method for widening and flattening the optical passband of such a device is also envisaged.